The present invention relates generally to xerographic copying methods, and more particularly to a contact fusing system for fixing toner material to a support substrate. In particular, the present invention relates to a method of fusing employing a novel toner release agent.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. The residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper with subsequent affixing or the image thereto.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalese and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. The silicone rubbers which can be used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e., room temperature vulcanization silicone rubber hereinafter referred to as RTV silicone rubber, low temperature vulcanization silicone rubber, referred to as LTV rubber, and high temperature vulcanization type silicone rubber, referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller and not penetrate into the substrate surface. This tackified material will stick to the surface of the fusing roller and come in contact with the subsequent sheet of supporting substrate bearing a toner image to be fused. A tackified image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tackified image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused therebetween toner may be transferred from the fuser roll to the pressure roll. This condition is referred to in the copying art as "offset". Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the abhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It has also been proposed to provide toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied on the fuser roll to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
In the two roll fusing systems wherein a silicone elastomer is used as the fuser surface, the silicone release oil typically has a viscosity of the order of 100 centistokes. This low viscosity enables the oil to be readily applied to the roll through a wicking process in a relatively easy manner to form the parting layer between the fuser roll and the image bearing surface. However, these low viscosity oils suffer from the difficulty in that being relatively low in viscosity, they are also relatively low in molecular weight, and thereby contribute to a swelling of the fuser roll by the migration or absorption of the silicone oil into the silicone rubber. Under certain conditions some small swelling may be acceptable, if it is uniform. However, the oil applied from the wick will be continuously removed by the paper but not removed outside the paper path. Thus, there will be a differential swelling between the areas inside and outside of the paper path. In addition, the passage of paper through the nip will cause a higher compression on the roll inside the paper path. Thus, there is a step created by swell on the roll at the 11 inch wide paper path. If the step height reaches about 3.0 mils and a 14 inch wide paper is now used, the toner along the 11 inch wide paper path edge will not be fused properly because of the step. This is referred to in the art as soft failure. The greater the rubber swells, the sooner the step will reach the critical failure dimension. In this way, rubber swell determines the soft failure life of a fuser roll.
Another type of failure occurs when rubber is delaminated from the core. This is known as hard failure. The exact mechanism is not clear but is believed to be due to the silicone oil diffusing through the rubber matrix to reach the core, where the silicone oil swelling can weaken the rubber at the locus of highest stress concentration and thereby cause delamination.
With the difficulties encountered in swelling of the fuser roll through the use of the low molecular weight and low viscosity release materials, it was first suggested to use the higher viscosity toner release agents to avoid this problem. Thus silicone oils having viscosities of up to say 60,000 centistokes were attempted. However significant difficulties were encountered in trying to handle this very high viscosity material. Particularly difficulties were encountered in trying to wick the material or deliver it from a supply source to the surface of the fuser roll. In addition the wicks have a tendency to clog with the high viscosity material and may even physically break down or shred.
Furthermore, since the wick is generally continuously engaged to the fuser member, a puddle of the silicone oil is created on the fuser roll. This puddle becomes excessively large with high viscosity silicone oils particularly during periods of idleness. Therefore after a period of idleness, the first copy fused contacts a fuser roll with a lot of oil on its surface which offsets to the copy paper which is objectionable. Furthermore, with the operation of the machine during a sequence of many short runs, the wicks are observed to dry out frequently since at each period of idleness they consume a lot of oil which is immediately taken up by the first few copies in the copy run. Thus with the concentration of oil in the wick required to pump the amount of oil necessary in the high viscosity type oils a much larger puddle was required. Furthermore, with the higher viscosity oils, the oils do not flow through the wick very rapidly and thus difficulties are encountered in the transportation of the oil from the supply to the operational surface. This is true because the high viscosity oil is much more difficult to move on a continuous basis. These high viscosity oils are manufactured as blends of other oils. A silicone oil having a viscosity of the order of 60,000 centistokes is made by blending separately made oils having viscosities of the order of 100,000 and 1,000 centistokes.
In addition to the above difficulties, the operational latitude of a fusing system employing the 60,000 centistokes oil is unduly restricted. By operational latitude it is intended to mean the difference in temperature between the minimum temperature required to fix the toner to the paper, the minimum fix temperature, and the temperature at which the hot toner will offset to the fuser roll, the hot offset temperature. Typically with the high viscosity 60,000 centistokes blended silicone oils, this operational latitude with a single paper is of the order of 60.degree.-70.degree. F. This has been determined to be too narrow for modern day reproducing flexibility which requires the capability to use many different types and weights of paper, different toner materials and amounts thereof, as well as respond to use in a wide variety of speeds and other operational conditions. It is also true that greater latitude is required to provide high quality copies particularly where toner pile height is increased to provide improved copy quality.